This invention relates to a control system for controlling the operation of a pressure-responsive fuel flow metering system. More specifically, this invention relates to a fuel flow metering control system capable for use as a Mach number hold unit for controlling fuel flow to jet engines used in aircraft.
Fuel flow devices and systems are widely available for governing fuel flow to jet engines used on aircraft. These fuel control devices normally are designed for controlling fuel flow under a specific set of operating conditions, such as either subsonic flight or supersonic flight, and in response to predetermined pressures or pressure ratios. For example, fuel control systems have been designed responsive to ram air pressure, such as those shown and described in U.S. Pat. Nos. 3,002,350; 3,026,673; 3,037,350; and 3,198,008. Other fuel control systems have been designed responsive to other flight pressures or parameters, such as ambient air pressure, engine inlet temperature, or other parameters which may be mechanically derived as by bellows and/or diaphragm arrangements, or by electronic computation elements. See, for example, U.S. Pat. Nos. 3,032,986; 3,156,291; 3,205,655; 3,530,666; 3,733,825; 3,813,063; and 3,938,322.
A major design problem in modern jet aircraft engines arises in providing highly accurate and consistent fuel metering control at high speed and high altitude cruise conditions, while at the same time providing satisfactory control at low speed and low altitude operation. A fuel metering control for closely holding aircraft speed at a constant absolute speed during high speed, high altitude cruise is desired for maximum fuel economy and passenger comfort. That is, particularly with modern wide body aircraft powered by relatively high bypass ratio fan-type engines, fuel flow is preferably held constant for smooth and efficient operation at absolute speeds on the order of about 0.80 Mach. However, fuel control systems designed for high speed, high altitude operation typically do not function satisfactorily at low speed and low altitude operation.
In the prior art, so-called Mach number hold units have been designed for controlling fuel flow at a relatively constant level during high altitude, high speed cruise conditions. These units of the prior art have basically comprised relatively conventional fuel flow metering control systems designed for operation under these specific cruise conditions, and have unfortunately included certain inherent limitations resulting in fuel flow inaccuracies and instabilities. For example, some of these systems comprise high gain systems responsive to ram air pressure, such as those systems shown in U.S. Pat. No. 3,021,670. However, while ram air pressure may be satisfactorily used at low speed and low altitude, fluctuations in ram air pressure tend to result in a gradual increase or creep of the fuel flow under high altitude conditions. Accordingly, to prevent this "creep", the flow control system must be adjusted manually, or by means of a relatively complex automatic pilot system. Other fuel flow control systems have attempted to provide alternate pressure-responsive systems for obtaining control pressures by means of restrictors or sonic nozzles, but these schemes too have failed to meet the high accuracy requirements of a Mach number hold unit. See, for example, U.S. Pat. Nos. 3,129,563 and 3,564,844.
The present invention overcomes the problems and disadvantages of the prior art by providing a fuel flow metering control system for use as a Mach number hold unit in jet aircraft engines wherein the system is responsive to a control pressure comprising a predetermined function of absolute ambient air pressure, as well as for use at low speed and altitude conditions wherein the system is responsive to ram air pressure.